The present invention relates to a semiconductor device, and more particularly, to a superjunction semiconductor device having an alternating conductivity type drift layer.
In general, a vertical type semiconductor device has a structure in which electrodes are arranged on two planes opposite to each other. When the vertical type semiconductor device is turned on, a drift current flows vertically in the semiconductor device. When the vertical type semiconductor device is turned off, depletion regions formed when a reverse bias voltage is applied to the device extend in the horizontal direction. To provide a high breakdown voltage to the vertical type semiconductor device, a drift layer disposed between the electrodes is formed of a material having high resistivity and the thickness of the drift layer is increased. In this case, however, an ON resistance of the device is also increased. This reduces conductivity and a switching speed, thereby degrading the operating characteristic of the device. It is well known that the ON resistance of the device is proportional to the breakdown voltage of the device to the power 2.5.
To solve this problem, a semiconductor device having a new junction structure has been recently proposed. This semiconductor device includes an alternating conductivity type drift layer composed of n regions (n pillar) and p regions (p pillar) alternately arranged. The alternating conductivity type drift layer forms a current path when the semiconductor device is turned on and is depleted when the semiconductor device is turned off. A semiconductor device having the alternating conductivity type drift layer is called a “superjunction semiconductor device”.
FIG. 1 is a layout of a conventional superjunction semiconductor device 100. Referring to FIG. 1, the superjunction semiconductor device 100 includes an active region 110 surrounded by an edge p pillar 120, and a termination region 130 surrounding the edge p pillar 120. While the edge p pillar 120 and the termination region 130 are separate from each other in FIG. 1, the edge p pillar 120 can be included in the termination region 130 under certain circumstances. The edge p pillar 120 is in the form of a rectangular ring with rounded corners. A plurality of active p pillars (not shown) and active n pillars (not shown) are alternately arranged in the horizontal direction in the active region 110. The active p pillars and the active n pillars form vertical strips. In addition, a plurality of termination p pillars (not shown) and termination n pillars (not shown) having the same form as the edge p pillar 120 are alternately arranged in the termination region 130, surrounding the edge p pillar 120.
In general, the superjunction semiconductor device 100 is designed to have a breakdown voltage larger in the termination region 130 than in the active region 110 because it is not desirable for breakdown to occur first in the termination region 130. To allow the superjunction semiconductor device 100 to have the higher breakdown voltage in the termination region 130 than in the active region 110, the difference in quantity of n charges and p charges is greater in the active region than in the termination region 130. However, these differences in quantity are very similar in the active region 110 and in the termination region 130. The quantity of n charges and the quantity of p charges must be balanced in both the active region 110 and the termination region 130 for the superjunction semiconductor device 100 to have satisfactory breakdown characteristic. However, the difference in the quantity of n charges and the quantity of p charges is greater in the upper part, lower part and corners of the edge p pillar 120, which come into contact with the active region 110, than in other parts. This deteriorates the breakdown characteristics of the superjunction semiconductor device.
FIG. 2 illustrates a corner and a portion of the upper part of the superjunction semiconductor device 100 of FIG. 1. Referring to FIG. 2, the active p pillars and the active n pillars are arranged in a region outside a corner region C, and upper and lower parts of the active region 110 such that the quantity of p charges in the active p pillars and the quantity of n charges in the active n pillars are equal. In the case of a unit cell A, for example, an active p pillar 111 having left and right regions 111-1 and 111-2 about a vertical central axis, an active n pillar 112, and an active p pillar 113 having left and right regions 113-1 and 113-2 about a the vertical central axis are arranged sequentially. Here, the sum (Qp1+Qp2) of the quantity of p charges Qp1 in the right region 111-2 of the active p pillar 111 and the quantity of p charges Qp2 in the left region 113-1 of the active p pillar 113 in the unit cell A and the quantity of n charges Qn1 in the active n pillar 112 disposed between the active p pillars 111 and 113 are equal. The balance of the quantity of charges is kept in all parts of the active region 110.
The termination p pillars and the termination n pillars are alternately arranged in the termination region 130 such that the quantity of p charges in the termination p pillar and the quantity of n charges in the termination n pillar are equal. In the case of a unit cell T shown in FIG. 2, for example, a termination n pillar 131 and a termination p pillar 132 are sequentially arranged outside the edge p pillar 120 having inner and outer regions 121 and 122 about a central axis. The termination p pillar 132 has inner and outer regions 132-1 and 132-2 about a central axis. Here, the sum Qpe+Qpt1 of the quantity of p charges Qpe in the outer region 122 of the edge p pillar 120 and the quantity of p charges Qpt1 in the inner region 132-1 of the termination p pillar 132 and the quantity of n charges Qnt in the termination n pillar 131 are equal. The balance of the quantity of charges is kept in all parts of the termination region 130.
However, the quantity of p charges and the quantity of n charges are severely unbalanced in the upper part, lower part and corners of the active region 110, which come into contact with the edge p pillar 120, because these parts do not have n charges that will be balanced with the p charges in the inner region 121 of the edge p pillar 120. More specifically, along vertical edges of the active region, the quantity of p charges in the inner region 121 of the edge p pillar 120 is equal to the quantity of charges in the active n pillars in the active region 110 parallel to the edge p pillar 120. The quantity of p charges and the quantity of n charges are balanced in the entire termination region 130 and the outer region 122 of the edge p pillar 120. However, the quantity of n charges in the inner region 121 in the corners, upper and lower parts of the edge p pillar 120 are not balanced, and surplus p charges exist. These surplus p charges break the balance between the quantity of p charges and the quantity of n charges in the corners, upper and lower parts of the edge p pillar and result in a deterioration of the breakdown characteristics of the superjunction semiconductor device.